The use of electrically charged fibrous materials as a filtration medium has been known for some time. Various methods and apparatus for preparing electrically charged webs and films are disclosed in U.S. patent application Ser. No. 07/958,958. Further disclosed in this U.S. patent application is the discovery that a suitable web or film may be conveniently cold charged by sequentially subjecting the web or film to a series of electric fields such that adjacent electric fields have substantially opposite polarities with respect to each other. Also disclosed are alternative apparatus for implementing such a process.
Generally speaking, such improvements are achieved by directing a web or film which is to be electrostatically charged between a coupled pair of conducting bodies. One of the conducting bodies can, if desired, serve to direct the web or film through the charging apparatus. The other conducting body combines to develop a potential difference such that ionization and corona occurs. This, in turn, creates an electric field (E) between the conducting bodies which is dependent upon the shape and the distance between the surfaces of the conducting bodies. This electric field is a vector (directional) quantity, and is generally measured in terms of volts per meter.
Such improvements are implemented with conducting bodies which include a charging bar spaced relative to either a charging drum (for receiving the web or film) or a charging shell (for overlying the web or film), which are appropriately biased relative to one another to develop the electric fields which are desired. While providing satisfactory results, it has been found that such apparatus is capable of improvement to achieve enhanced electrostatic charging of the webs and films which are to be treated.
To better understand the improvements of the present invention, a brief discussion of the mechanisms which contribute to ionization and corona is appropriate. To this end, as the above-discussed electric field is developed, charged particles are created in the region of the electric field by some appropriate process such as a cosmic ray or some other natural radiation source. Each particle is accelerated by the electric field along the vector direction which is defined by the electric field. If sufficient energy is gained by the charged particle before that charged particle collides with a gas atom, an electron is removed from the atom and is made available for acceleration and further ionizations.
This breakdown process is characterized by several "loss" mechanisms. This includes:
1) capture of electrons by heavy oxygen atoms, PA1 2) collision with the electrodes' "current flow", and PA1 3) movement of particles out of the electric field.
If the first and third loss mechanisms are large in comparison to the number of charged particles which are created, the result is what is known as a "corona current" (i.e., a low current flow at a very high voltage). However, this type of current flow is very unstable, and is likely to lead to arcing. If the second loss mechanism becomes dominant, the result is an arc, which is both uncontrollable and destructive to the end product. Clearly, such an occurrence is to be avoided.